Over the past twenty years, rare earth doped optical fibers have had a tremendous impact on the laser industry. The first application that deployed a significant volume of rare earth doped optical fiber based devices was optical amplification in the telecommunications industry. In this application, optical signals carrying data are sent through single mode fiber typically at a wavelength near 1.5 microns. As these signals propagate over long distances, they are attenuated due to scattering and absorption losses in the transmission fiber. By coupling these transmission fibers to a section of Erbium doped fiber that is pumped with a wavelength near 980 nm or 1480 nm, these signals can be amplified back to their original intensity level. These devices are commonly known as erbium doped fiber amplifiers or EDFAs.
When compared with other lasers and optical amplifiers, fiber based devices typically offer higher gain and higher overall efficiency. As the average power levels, pulse energies and peak powers of fiber lasers and fiber amplifiers continue to increase, rare earth doped optical fibers have begun to be used in a far broader range of applications. These applications are found in the medical, industrial, defense, and semiconductor processing industries.
Continued expansion of fiber laser applications requires further increases in average power, pulse energy and peak power. Increasing the average power of fiber lasers is largely driven by the brightness of laser diode pumps and the ability to couple power into fiber. Pulse energy and peak power on the other hand are respectively driven by the ability to store and extract energy in the fiber while mitigating the nonlinear processes than can have adverse impacts on the temporal and spectral content of the output pulse. Both of these issues can be effectively addressed by using fibers with large core sizes.
The fundamental transverse mode of an optical fiber has very desirable characteristics in terms of beam shape, minimum beam expansion during propagation through free space (often referred to as “diffraction limited”) and optimum focus-ability. However, as a result, most applications benefit greatly from single mode or fundamental mode operation of fiber lasers and amplifiers. As the core size of an optical fiber is increased, the fiber begins to support the propagation of more than one transverse optical mode. The number of modes supported by an optical fiber can be roughly calculated by using the fiber's so-called V-number. The V-number of a fiber is defined asV=2πa/λNAwherein a is the diameter of the fiber core and NA is the numerical aperture of the core. The number of modes supported by the fiber is given by roughly one half the square of the V-number. It can be shown that a fiber with a V-number less than about 2.4 supports the propagation of only the fundamental mode.
In a typical high peak power, high pulse energy fiber amplifier today, the core might have an aperture of 25 microns and an NA of 0.07 giving a V-number of over 10. Such a fiber supports the propagation of several higher order modes. Achieving fundamental mode output from an amplifier using such a multimode fiber therefore requires either a method to prevent the excitation of higher order modes or to remove the higher order modes from the light propagating in the fiber.
In U.S. Pat. No. 5,818,630, an approach is disclosed wherein a near diffraction limited seed source is optically coupled to a multi-mode fiber amplifier. Through the use of a mode-converter, defined as either a set of bulk lenses or a tapered section of fiber, the beam size is changed to match as nearly as possible that of the fundamental mode of the multimode amplifier fiber.
It is generally difficult to excite only the fundamental mode of the amplifier fiber even if a mode convertor is used. The fundamental mode of most seed lasers is not the same as the fundamental mode of an optical fiber. For this reason, even with a mode-converter described in U.S. Pat. No. 5,818,630, higher order modes of the multimode optical fiber will typically be excited to some extent.
Further, any changes in launch conditions that result, for example, through optic movements that can be induced by the operating environmental conditions such as, for example, vibration or changes in temperature, can alter the amount of seed power coupled into each of the numerous optical modes of the fiber. This causes the corresponding changes in output beam shape and mode quality.
The inventors have also observed that when higher order modes are excited in multimode fiber the output beam shape and mode quality is highly sensitive to both micro and macro bends in the fiber. Even with a stable package that prevents changes in micro and macro bending of the fiber during operation, this sensitivity makes manufacturing of the device challenging as the output is not stable.
These issues limit the utility of this amplifier configuration for many applications.
In U.S. Pat. No. 6,496,301, which is incorporated herein by reference, discloses an optical amplifier that enables embodiments where the coupling of seed light into a multimode amplifier results in the excitation of higher order modes. To prevent these higher order modes from impacting the output beam, this invention tightly coils the amplifier fiber. This approach results in a distributed bend induced loss that strips the power from the higher order modes in the amplifier fiber.
The bend induced loss is a relatively strong function of the spatial order of the fiber mode. For modes that are radially symmetric, the loss is independent of the axis of the coil with higher order modes experiencing higher loss. For modes that are radially asymmetric, the loss is dependent on the axis of the coil. To ensure sufficient loss for all modes, it is therefore sometimes required to coil the fiber about one axis followed by a coil on an axis oriented at 90 degrees with respect to the first one. Amplifiers made in this way can be designed to operate stably on only the fundamental mode.
Unfortunately, there are also other practical limitations associated with this design. One limitation is that to strip the higher order modes effectively, some loss is also created for the fundamental mode. This distributed loss for the fundamental mode limits the overall efficiency of the fiber amplifier. Another limitation is that light lost from the core to bend losses is captured by the pump cladding. This light then exits the output end of the fiber and results in a halo of light surrounding the main output beam. Yet a third limitation associated with this design is that as the fiber is bent, the effective mode area of the fundamental beam is reduced. This increases the irradiance of the signal within the fiber and, as described earlier, leads to increased amounts of nonlinear effects. These nonlinear effects can limit the peak power capability of the fiber amplifier.
It is clear that further improvements in peak power require an improved fiber amplifier.